An automatic transmission for a vehicle provided with a fluid-type torque converter has been widely known. However, such fluid-type torque converter may suffer from slippages when transmitting power, leading to a loss of transmission efficiency. In light of the foregoing, automation of a transmission apparatus that includes a gear-type manual transmission has been suggested. For example, as disclosed in JP64-46046A (Reference 1, 10th line from the lower left in page 2 to 17th line from the upper left in page 3, FIGS. 1 to 7), a portion of each shift fork engages with each cam groove formed at an outer periphery of a cylindrical cam so that a shift operation can be performed by a rotation of the cylindrical cam. In addition, as disclosed in JP2006-002789A (Reference 2, paragraphs 3 to 9, FIG. 8), a dual clutch transmission is equipped with an automatic clutch mechanism of two frictional clutches that are brought to an engagement state and a disengagement state in turns, i.e., a dual clutch mechanism.
According to a cylindrical cam-type automatic operating mechanism of the transmission disclosed in Reference 1, multiple cam grooves are formed at the outer periphery of the cylindrical cam that is driven to rotate by a motor. The portion (i.e., shift lever) of each shift fork by means of which gears of the transmission are shifted engages with each cam groove so that the shift fork performs a reciprocating movement. When the cylindrical cam is driven to rotate by the motor, the shift operation is performed in such a way that, while one of the shift forks is being driven, the remaining shift forks that are not being driven are operated in association with one another and stopped in neutral positions. According to the disclosed transmission, multiple actuator functions can be achieved by a single cylindrical cam and thus only one motor is required, which leads to a downsized and light-weighted transmission.
According to the dual clutch transmission disclosed in Reference 2, an input, which is transmitted from a drive shaft such as an output shaft of an engine, is first transmitted to a first input shaft via a first frictional clutch of the dual clutch mechanism, and then is transmitted to a second input shaft of a hollow shape. The second input shaft is arranged at a peripheral portion of the first input shaft so as to be coaxial with the first input shaft. As being explained in FIG. 5, the first and second frictional clutches of the dual clutch mechanism are controlled in a manner that, during a shift operation, when a transfer torque A increases, a transfer torque B decreases in contrast with the transfer torque A, and vice versa. After the shift operation, the torque transfer A reaches the maximum torque value T0 and the transfer torque B falls down to zero, and vice versa. This dual clutch transmission includes first and second counter shafts arranged in parallel with the first and second input shafts. Arranged between the first input shaft and the first and second counter shafts is a first gear change mechanism including four gear trains for a first shift stage, a third shift stage, a fifth shift stage, and a seventh shift stage, respectively. Arranged between the second input shaft and the first and second counter shafts is a second gear change mechanism including three gear trains for a second shift stage, a fourth shift stage, and a sixth shift stage. Further, arranged between the first input shaft and the second counter shaft is a gear train for a reverse shift stage. In the operation of the dual clutch transmission, a controller thereof controls the first frictional clutch and the second frictional clutch to be engaged or disengaged in turns. The operations of the first and second frictional clutches are responsive to a condition of a vehicle, such as an accelerator opening degree, an engine rotational speed (rpm), a speed of the vehicle, or the like. When the first frictional clutch is controlled to the engagement state, one of the 1st, 3rd, 5th, and 7th shift stages is selected and power transmission in accordance with the selected shift stage is implemented. On the other hand, when the second frictional clutch is controlled to the engagement state, one of the 2nd, 4th, 6th, and reverse shift stages is selected and power transmission in accordance with the selected shift stage is implemented.
In the cases where the cam-type automatic operating mechanism as disclosed in Reference 1 is applied to the dual clutch transmission mechanism as disclosed in Reference 2, the multiple actuator functions can be achieved by a single cylindrical cam so that the number of motors required can be reduced or only one motor can be required, which leads to a downsized and light-weighted transmission. FIGS. 10 and 11 each illustrate a structure in the vicinity of a shifting device of such dual clutch transmission as a comparison example. This dual clutch transmission of the comparison example includes a first gear change mechanism 20A constituted by two gear trains each of which consists of a driving gear and a driven gear (in FIG. 11, only driven gears 21b and 23b are illustrated) arranged between a first input shaft (not shown) and a counter shaft 15, and a first switching clutch 30A provided at the counter shaft 15 so as to select the power transmission between the first input shaft and the counter shaft 15 conducted by the aforementioned gear trains. The dual clutch transmission also includes a second gear change mechanism 20B constituted by two gear trains each of which consists of a driving gear and a driven gear (in FIG. 11, only the driven gears 22b and 24b are illustrated) arranged between a second input shaft (not shown) and the counter shaft 15, and a second switching clutch 30B provided at the counter shaft 15 so as to select the power transmission between the second input shaft and the counter shaft 15 conducted by the aforementioned gear trains. A shift mechanism 1A includes first and second shifters 2A and 2B, and a cylindrical cam 5A. The first and second shifters 2A and 2B include body portions 2a and 2d, respectively, which are axially slidably guided and supported by a guide bar 3A that is arranged in parallel with the counter shaft 15. The cylindrical cam 5A is arranged in parallel with the counter shaft 15 in a rotatable manner. A shift fork 2b and a follower pin 2c integrally formed with the body portion 2a of the first shifter 2A engage with a peripheral groove of a sleeve M provided at the first switching clutch 30A, and a cam groove 6A formed at an outer peripheral surface of the cylindrical cam 5A, respectively. In the same way, a shift fork 2e and a follower pin 2f integrally formed with the body portion 2d of the second shifter 2B engage with a peripheral groove of a sleeve M provided at the second switching clutch 30B, and the a cam groove 6B formed at the outer peripheral surface of the cylindrical cam 5A, respectively.
When the cylindrical cam 5A is driven to rotate by a shift motor (not shown) via a worm wheel 7A and a worm 8A, then the first and second shifters 2A and 2B of which respective follower pins 2c and 2f are engaging with the cam grooves 6A and 6B are driven to reciprocate along the guide bar 3A. In response to this reciprocating movement of the first and second shifters 2A and 2B, the shift forks 2b and 2e cause the respective sleeves M of the first and second switching clutches 30A and 30B to perform a reciprocating movement. Then, the first and second gear change mechanisms 20A and 20B are activated to thereby achieve a gear shift operation.
According to the aforementioned shift mechanism of the dual clutch transmission, the first and second shifters 2A and 2B are operated by the single cylindrical cam 5A. Thus, the downsized and light-weighted transmission can be achieved. However, a length of the cylindrical cam 5A is large because of the two cam grooves 6A and 6B formed and thus downsizing and weight reducing the shifting device may be insufficient.
Thus, a need exists for a shifting device for a dual clutch transmission having a dual clutch mechanism that can be further downsized and reduced in weight and that requires only a small operation force by means of a single cam groove formed at a cam member such as a cylindrical cam.